I purchased a rebuilt 1912 Steinway B four months ago. It has been tuned three times since I got it. Each time, it seems like after playing 20-30 minutes the intervals start to migrate. Perhaps it is overtones or beats but it’s enough to be distracting to me. Two technicians have said the soundboard and bridges were well done. I think the action has older parts but for the life of me I cannot figure out why this piano doesn’t stay in tune better or have a better tone. Any thoughts?